1. Field of Invention
This invention relates to methods and apparatus for well logging. More specifically, this invention relates to a method and apparatus for determining parameters of acoustic waves to investigate an acoustic medium within the sphere of influence of an acoustic borehole logging tool.
2. Background of the Invention
Acoustic well logging techniques and tools are extensively described in the art. Acoustic well logging is used to provide surveys of formations traversed by earth boreholes. In particular, measurements are made of the velocities of acoustic waves to reveal valuable information concerning the type of rocks and the porosity of the rocks in the formations surrounding the borehole. A commonly measured acoustic parameter is the velocity of compressional waves. However, other acoustic wave parameters such as the velocity of shear waves and velocities of other modes of acoustic energy are also useful in evaluating the formation.
Identifying the compressional wave and measuring its velocity is generally not difficult. It is the fastest propagating wave in the formation, is non-dispersive, and is the first to reach an array of borehole receivers when a short burst of energy from a nearby transmitter propagates through the formation. By measuring the arrival times of these waves at the receivers, the wave velocity in the vicinity of the array can be inferred.
Measuring shear velocity is considerably more difficult. Because it propagates more slowly through the formation the shear wave arrives later in time. Therefore, its arrival is typically obscured by compressional energy, and velocity determination directly from arrival time then becomes impossible.
An acoustic well logging tool for measuring the velocity of acoustic waves typically employs a sonic pulse transmitter and a plurality of sonic receivers selectively spaced from the transmitter. The sonic receivers include transducers to convert the incident acoustic wave to an electrical waveform and suitable amplifiers to transmit the waveforms to surface located processing equipment.
The spacings between the sonic receivers and the transmitter influence the character of the parameter measurement. For example, a longer spacing enables a greater effect of the formation characteristics on the sonic wave with a possibility that wave analysis can thus reveal more useful data about the formation. On the other hand, a longer spacing results in the arrival of greatly attenuated sonic waves making their analysis more difficult. Current sonic well logging tools commonly use one transmitter with a plurality of sonic receivers. A double pair of transmitter-receivers has been used in the art, for example to provide compensation for a physical misalignment of the tool in the borehole as described in U.S. Pat. No. 3,304,537 to Schwartz. A three receiver, single transmitter sonic well logging tool has been described in U.S. Pat. No. 3,390,377 to Elliott et al and U.S. Pat. No. 4,210,966 describes a technique for determining parameters of acoustic waves employing a system having a transmitter and four receivers. Generally, when the number of receivers is increased to improve the velocity measurement, special techniques are needed to process the increase in the flow of signals and enhance the accuracy of sonic wave parameter measurements.
The prior art further discloses several methods for determining acoustic wave parameters in well logging operations. One such method, a first-motion detection, is particularly applicable to measuring the transit time of the formation compressional waves. Another technique, multifold correlation, has been applied to segments of compressional wave forms detected at various spaced positions to refine the results of the first motion detection. It has also been used for determining other acoustic parameters such as shear wave velocity. Additional techniques include pattern comparison, measurement of correlation between waveform segments as a function of wave form locations, and, particularly in the seismological field, frequency and phase analysis to detect speed and direction of propagation.
While such methods furnish good information about the characteristics of the earth formations being measured, they generally require assumptions of parameters or earlier measurements such as of compressional velocity to correctly estimate the velocity of such wave components as shear waves and the velocity of modal waves. Many of the techniques also do not work well in broad ranges of lithologies, or are difficult to interpret. A need therefore remains for a more accurate, more versatile and more reliable method and apparatus for compressional, shear and modal acoustic velocity borehole logging.